GAE / JPA / Datastore如何查询未拥有的列表

Question

I am trying to do something simple I thought but it is not working as expected using the Big Table and JPA, I am using datanucleus jpa v2.

@Entity
public class Inventory extends DatastoreObject {
...

    /**
     * List of all inventory items in this object.
     */
    @OneToMany(mappedBy = "inventory", fetch = FetchType.LAZY, cascade={CascadeType.ALL})
    private List<InventoryItem> inventoryItems = new ArrayList<InventoryItem>();

...

}



@Entity
public class InventoryItem extends DatastoreObject {

    ...

    @ManyToOne
    private Inventory inventory;

    ...    
}

When I look at the Datastore Viewer, I see entities with their correlating relationship columns. This is the inventory table which has a list of ids for the inventory items.

Inventory Table
    Key    Write Ops    ID/Name         inventoryItems 
    agxzbWFydGJhcnNpdGVyDwsSCUludmVudG9yeRgwDA  8   48      [InventoryItem(69)] 


Inventory Item Table
    Key    Write Ops    ID/Name     inventory_id
    agxzbWFydGJhcnNpdGVyEwsSDUludmVudG9yeUl0ZW0YRQw     14  69  Inventory(48)

My question is how do I fetch all the inventory item's where their inventory_id is for example 48. I know I can fetch the inventory object and do an array sublist but this seems really inefficient.

When I try to do a query it doesn't work as expected and I understand some of the reasons as joins are not supported but it seems that it would work if I could access the inventory_id column of then inventory item table but it seems that I cannot.

When I try simple queries this won't work.

"Select from InventoryItem.class.getName() item where item.inventory.id = :inventoryId"

I was hoping this would work in datanucleus, I understand that it is typically a join but if datanucleus knows that it is an unowned relationship and that the inventory object is mapped by it's id couldn't this work somehow? I understand that the .id attribute wouldn't make any sense as it is an arbitrary token but it seems there should be a way to do this type of query with an unowned @OneToMany(mappedBy="inventory") configuration.

I tried the other way also by assigning the object as a parameter but to my surprise this did not work either.

"Select from InventoryItem.class.getName() item where item.inventory = :inventory"

Here I try the query by first fetching the object and then trying to query using that object as the parameter but again this did not work unless the objects was "embedded"

My preferred solution would be the first query operation but I am pretty sure this is not possible using big table. I am still confused though why the second operation does not work.

Any help would be appreciated.

Answer 1

Well in such cases what I prefer instead of putting any relationships between entities, I simply create Key for one of my Entity and I save the same key in the other entity as a Foreign Key, so if I need to query the child entity I use Foreign Key as get back my results.

I have recently started using Objectify and I suggest you that if you need such kind of problems you can use objectify to persist data to Appengine Datastore. It is very efficient and simple to learn and use.

Answer 2

Is that JPA query ok? It should look like this:

select item from InventoryItem item where item.inventory.id = :inventoryId